mastering ch.13

You may remember this shot from the video pod cast. Here I am representing a duplicated chromosome. What structure is represented by my waist, where I am joined?

centromere [The centromere is the point of attachment between two sister chromatids of a duplicated chromosome.]

In alternation of generations, what is the diploid stage of a plant that follows fertilization called?

sporophyte [The sporophyte is the diploid, multicellular stage of the plant that produces haploid spores by meiosis.]

Independent assortment of chromosomes during meiosis is a result of

the random and independent way in which each pair of homologous chromosomes lines up at the metaphase plate during meiosis I.

What stage of the life cycle must this cell be in?

the start of meiosis 1 [We can see that the chromosomes are duplicated and lined up by homologous pair. That only happens at the start of meiosis I.]

Identify all possible products of meiosis in plant and animal life cycles.

-Spores -Gametes (sperm and eggs) In most animal life cycles, the products of meiosis are gametes. However, in plants, the products of meiosis are spores.

Imagine you found a hypothetical organism. You examine one of its gametes and you see that it contains 5 chromosomes. How many chromosomes will one of its body cells contain just before mitosis begins?

10 [If the hypothetical organism has 5 chromosomes in one of its gametes, then it must have 10 chromosomes in a body cell. Just before mitosis, the chromosomes in a body cell are duplicated, but the sister chromatids remain joined together. So you will see 10 (duplicated) chromosomes in a typical body cell.]

PART B - Independent assortment and genetic variation Consider a diploid cell where 2n = 6. During metaphase I of meiosis, as the pairs of homologous chromosomes line up on the metaphase plate, each pair may orient with its maternal or paternal homolog closer to a given pole. There are four equally probable arrangements of the homologous pairs at metaphase I. (Note that this problem assumes that no crossing over has occurred.) The cells below show the eight possible combinations of chromosomes that the daughter cells of meiosis II can receive. Sort each daughter cell into the appropriate bin depending on which arrangement at metaphase I would create it.

ARRANGEMENT 1: 3 & 8 ARRANGEMENT 2: 1 & 5 ARRANGEMENT 3: 6 & 2 ARRANGEMENT 4: 4 & 7 [One aspect of meiosis that generates genetic variation is the random orientation of homologous pairs of chromosomes at metaphase I. Each pair can orient with either its maternal or paternal homolog closer to a given pole; as a result, each pair sorts into daughter cells independently of every other pair. Due to independent assortment alone, a diploid cell with 2n chromosomes can produce 2^n possible combinations of maternal and paternal chromosomes in its daughter cells. For the cell in this problem (n=3), there are 2³, or 8, possible combinations; for humans (n = 23), there are 2²³, or 8.4 million, possible combinations. Note that when crossing over occurs, the number of possible combinations is even greater.]

PART C - Changes in ploidy and DNA content during meiosis The parent cell that enters meiosis is diploid, whereas the four daughter cells that result are haploid. Which statement correctly describes how cellular DNA content and ploidy levels change during meiosis I and meiosis II?

DNA content is halved in both meiosis I and meiosis II. Ploidy level changes from diploid to haploid in meiosis I, and remains haploid in meiosis II. [During anaphase of both meiosis I and meiosis II, the DNA content (number of copies of chromosomes) in a cell is halved. However, the ploidy level changes only when the number of unique chromosome sets in the cell changes. This occurs only in meiosis I (where separation of homologous chromosomes decreases the ploidy level from 2n to n and produces daughter cells with a single chromosome set).]

PART A - Processes that determine heredity and contribute to genetic variation Meiosis guarantees that in a sexual life cycle, offspring will inherit one complete set of chromosomes (and their associated genes and traits) from each parent. The transmission of traits from parents to offspring is called heredity. Another important aspect of meiosis and the sexual life cycle is the role these processes play in contributing to genetic variation. Although offspring often resemble their parents, they are genetically different from both of their parents and from one another. The degree of variation may be tremendous. The following processes are associated with meiosis and the sexual life cycle: -DNA replication before meiosis -crossing over -chromosome alignment in metaphase I and separation in anaphase I -chromosome alignment in metaphase II and separation in anaphase II -fertilization Sort each process into the appropriate bin according to whether it contributes to heredity only, genetic variation only, or both. (Note that a bin may be left empty.)

HEREDITY ONLY: none GENETIC VARIATION ONLY: none BOTH: all -fertilization -metaphase 1→anaphase 1 -metaphase 2→anaphase 2 -crossing over -DNA replication [In organisms that reproduce sexually, the processes of DNA replication, the precise pairing of homologs during crossing over, chromosome alignment and separation in meiosis I and II, and fertilization ensure that traits pass from one generation to the next. Unlike with asexual reproduction, offspring of sexual reproduction are genetically different from each other and from both of their parents. Mechanisms that contribute to genetic variation include -errors (mutations) that occur during DNA replication -the production of recombinant chromosomes due to crossing over -the independent assortment of homologous chromosomes in meiosis I -the separation of sister chromatids (which are no longer identical due to crossing over) in meiosis II -the random fusion of male and female gametes during fertilization]

Which of the following occurs during meiosis but not during mitosis?

Synapsis occurs. [The pairing of homologous chromosomes that only occurs during prophase I of meiosis is called synapsis.]

PARTA - The stages of meiosis Can you recognize the eight stages of meiosis based on the location and behavior of the chromosomes? Drag the diagrams of the stages of meiosis onto the targets so that the four stages of meiosis I and the four stages of meiosis II are in the proper sequence from left to right. (Note that only one of the two daughter cells is shown for meiosis II.)

[Meiosis involves two sequential cellular divisions. In meiosis I, homologous chromosomes pair and then separate. Thus, although the parent cell is diploid (containing two chromosome sets, one maternal and one paternal), each of the two daughter cells is haploid (containing only a single chromosome set). In meiosis II, the sister chromatids separate. The four daughter cells that result are haploid.]

PART C - Crossing over and genetic variation Assume that an organism exists in which crossing over does not occur, but that all other processes associated with meiosis occur normally. Consider how the absence of crossing over would affect the outcome of meiosis. If crossing over did not occur, which of the following statements about meiosis would be true? Select all that apply.

There would be less genetic variation among gametes. [Crossing over contributes significantly to the genetic variation seen in gametes. This is because the exchange of maternal and paternal genes between the nonsister chromatids of a homologous chromosome pair creates recombinant chromosomes with unique combinations of alleles. However, crossing over is not the only process that introduces genetic variation in meiosis I. The independent assortment of homologous chromosomes (which are never identical) in meiosis I produces daughter cells that differ from each other. The effect of crossing over on genetic variation is shown below. Without crossing over, sister chromatids remain identical and thus, pairs of daughter cells would be identical. With crossing over, however, all four daughter cells are genetically unique.]

Look at the cell in the figure. Based on this figure, which of the following statements is true?

This cell is diploid. [This cell contains two copies of each chromosome, one from the male parent and the other from the female parent, making it diploid.]

PART B - Crossing over Crossing over plays a critical role in increasing the genetic variation among offspring of sexual reproduction. It is important to understand how crossing over occurs and its consequences in meiosis. Look carefully at the diagrams depicting different stages in meiosis in a cell where 2n = 6. Assume that the red chromosomes are of maternal origin and the blue chromosomes are of paternal origin. Drag the labels to fill in the targets beneath each diagram of a cell. Note that the diagrams are in no particular order. Drag the blue labels to the blue targets to identify the stage of meiosis depicted in each diagram. Drag the pink labels to the pink targets to identify whether the configuration of the chromosomes related to crossing over is possible or not.

[Crossing over occurs during prophase I when homologous chromosomes loosely pair up along their lengths. Crossing over occurs only between nonsister chromatids within a homologous pair of chromosomes, not between the sister chromatids of a replicated chromosome. Only segments near the ends of the chromatids, not segments nearest the centromeres, can exchange DNA.]

PART A - Meiosis terminology Drag the labels from the left to their correct locations in the concept map on the right.

[Knowing the terms and relationships shown in this concept map will help you understand the role that meiosis plays in heredity, sexual reproduction, and genetic variability.]

PART C - Animal life cycles In the life cycle of an organism, meiosis is paired with the process of fertilization. Understanding the life cycle of an organism is the key to understanding how sexual reproduction ensures the inheritance of traits from both parents and also introduces genetic variation. Complete the diagram to show the life cycle of a typical animal. Follow these steps: 1. First, drag blue labels onto blue targets only to identify each stage of the life cycle. 2. Next, drag pink labels onto pink targets only to identify the process by which each stage occurs. 3. Then, drag white labels onto white targets only to identify the ploidy level at each stage. Labels can be used once, more than once, or not at all.

[Meiosis creates gametes (eggs and sperm) with only a single chromosome set (haploid or n) from parental cells with two chromosome sets (diploid or 2n). During fertilization, the haploid sperm (n) and egg (n) fuse, producing a diploid zygote (2n). The cells of the zygote then divide by mitosis (which does not change the ploidy level) to produce an adult organism (still 2n) of the next generation. In sexual life cycles, meiosis and fertilization keep the number of chromosomes constant from generation to generation.]

PART B - Interactions among chromosomes This diagram shows a diploid nucleus (2n=8) in which chromosome replication has occurred in preparation for mitosis (top) and meiosis (bottom). The nucleus at top right is now in prophase of mitosis; the nucleus at bottom right is now in prophase I of meiosis. Drag the labels to their appropriate targets to correctly identify the various chromosome structures. Labels can be used more than once.

[To understand the process of meiosis, it is essential that you can differentiate between sister chromatids, nonsister chromatids, homologous chromosomes, and non-homologous chromosomes.]

If you look through a microscope and see a cell with chromosomes lined up two by two, what stage of cellular reproduction must you be looking at?

meiosis 1 only [Meiosis I is the only time that chromosomes line up by homologous pairs.]

Of the two processes we have learned about, which is going on in my hand right now?

mitosis [Mitosis goes on continuously in most parts of the body, whereas meiosis takes place only in the gonads.]